In certain types of computing systems, it is useful to know which of several, usually similar, component devices in the system has gone the longest without being used. For example, if the component devices are main memories, cache memories, or memory areas storing pages in a virtual memory computer system, when a new page not in one of the memories is requested, it is common practice to use the memory or memory area which has gone the longest without reference as the repository for the new page. This has become known as the least recently used (LRU) algorithm. Because time is quite important in resolving LRU status, dedicated hardware is frequently employed for this purpose. Typical hardware of this type is shown in Computer Storage Systems and Technology, p. 596, Richard Matick, 1977, John Wiley & Sons.
With certain types of new memories, a trade-off of high speed and low cost is made versus reliability. In such a situation, if one memory fails and is not used, the other devices can be used with greater frequency and somewhat lower overall speed until the faulty device can be replaced or repaired. Such a faulty memory should not be considered in resolving the LRU algorithm, even though it will in fact, through its deliberate non-use, become the LRU device shortly after it becomes faulty. Accordingly, it is necessary to modify or disable the conventional LRU resolving network to ignore the not-to-be-used device.